1. Field of the Invention
The invention relates to a method for filling pressure vessels, in particular pressure vessels in airbag systems, with gas.
2. Description of the Related Art
Airbags in vehicles are increasingly using new gas generators which inflate the airbag within a few milliseconds in the event of an accident. At present, three types of gas generators are in use:                chemical generators, in which the gas is generated by the reaction of a chemical solid with ambient air;        what are known as hybrid generators, which comprise a combination of solid fuel and compressed-gas packing; and        pure gas generators with a high-pressure gas storage system with pressures of up to 700 bar at 15° C.        
The gas generators, which are filled with various gases, present huge technical problems in production, both in terms of their own production and during filling with pressures of up to 1000 bar. These pressures are required in particular for rapid filling on account of the heat of compression in order for the accurately predetermined masses of gas to be introduced. These masses are of crucial importance to the subsequent inflation characteristics of the airbag.
Examples of gases used include argon, oxygen, nitrogen, dinitrogen monoxide (laughing gas), both in the form of ultrapure gases and in the form of gas mixtures made up of these components.
Demands imposed on gas-filled gas generators are:    1. Filling pressures of up to 1000 bar (P(T)) for a higher storage density or more compact overall dimensions.    2. Accurate filling quantity determination at high pressures.    3. Rapid filling, since it is this operation which determines the cycle times.    4. The operation must be highly reproducible.
Very expensive and complex piston or diaphragm compressors are required in order to generate the very high pressures. This leads to high investment costs, high operating and maintenance costs. In addition, a correspondingly complex and expensive downstream gas supply is required for these pressures.
On account of the heat of compression and the uneven temperature distribution in the pressure vessel, the inaccuracy of the precise filling quantity determination rises with increasing pressures, yet accurate determination is imperative for the subsequent defined functioning of the generator.
As pressures increase, it becomes technically more difficult and expensive to achieve fast filling times. There is a direct relationship between filling time and warming during the filling operation, i.e. the more quickly filling takes place, the greater the rise in gas temperature, with the result that this filling pressure has to be increased still further in order to achieve the precise quantity of gas for 15° C. or another defined temperature.
The reproducibility becomes more difficult for the reasons mentioned above and entails complex QA measures, such as for example weighing of the filled vessels for accurate filling quantity determination. At the same time, a significant rise in scrap rates is expected at higher pressures. This in turn adversely affects the economics of the entire process and therefore leads to higher production costs.